With atmospheric carbon dioxide concentrations at new record highs and global average temperature now some 0.8 degrees Celsius above pre-industrial levels, the frozen regions of the earth are showing us just how rapidly climate change can take effect. Recent years have seen ice melt accelerate and spread to new, previously unaffected regions. In many areas, the pace of melting has surprised even the scientists studying it most closely, providing a strong early indication that the consequences of climate change could come faster and be more severe than previously believed. (See larger image of map.)

The most dramatic loss of ice in recent years has been the decline of summer sea ice in the Arctic Ocean. Between 1953 and 2006, the area covered by sea ice in September shrunk by 7.8 percent per decade, more than three times as fast as the average rate simulated by climate models. Researchers were further stunned in the summer of 2007 when Arctic sea ice extent plummeted to the lowest level ever measured, more than 20 percent below the 2005 record. This decline is rapidly changing the geopolitics of the Arctic region, opening the Northwest Passage for the first time in recorded history and triggering a scramble among governments to claim large swaths of the potentially resource-rich Arctic sea floor.

An important factor behind the sudden drop in ice cover in 2007 was that the sea ice at the start of the spring melt-season was thinner and less extensive than usual. The fact that the ice was unable to fully recover over winter has led researchers to suggest that a tipping point has already been reached: many now believe the summer Arctic Ocean could be ice-free by 2030, decades earlier than previously thought possible. Arctic sea ice both reflects sunlight and acts as an insulating layer between the relatively warm ocean and the colder atmosphere. As it melts away, these cooling effects disappear, warming the region still further. Sea-ice decline may therefore be part of the reason why average Arctic temperatures have risen at almost twice the global rate in the last 100 years.

Warmer temperatures are also accelerating ice melt on the nearby Greenland ice sheet, which contains enough ice to raise sea level by seven meters (23 feet). Mass loss in Greenland more than doubled between 1996 and 2005, with loss in the southeast accelerating even further since 2004. The summer of 2007 saw a record area of ice melt on Greenland, 10 percent more than the previous maximum in 2005.

In 2006, scientists reported that “glacial earthquakes” caused by large masses of ice moving rapidly over bedrock had doubled in frequency in Greenland over the last five years. These earthquakes are associated with meltwater from the glacier surface, which flows to the base of the ice sheet and lubricates it, causing rapid glacial movement. Positive feedback mechanisms such as this meltwater lubrication accelerate the speed with which glaciers react to warmer temperatures; ice sheets once thought to change only over millennia are now seen to be responding to warmer temperatures in just decades.

At the other end of the earth, the West Antarctic Ice Sheet (WAIS) is also showing disturbing early signs of disintegration. These include the thinning and acceleration of glaciers near the coast, the retreat of grounding lines (the point at which glaciers leave the land and become floating ice shelves), and the increased calving of large icebergs. A recent study estimates that West Antarctica is losing approximately 132 billion tons of ice per year, 59 percent faster than only a decade ago. In 2007, researchers reported satellite data showing large lakes and rivers of meltwater flowing beneath the ice sheet, suggesting that the positive-feedback mechanisms recently found to be accelerating ice loss in Greenland are also at work in Antarctica.

The WAIS is thought to be particularly vulnerable to warming because its base rests largely below sea level; higher sea level or a warmer ocean could lead to an unstable retreat of the grounding line toward the interior, producing a sudden and rapid disintegration of the ice sheet. The Pine Island Bay area of the WAIS near the Amundsen Sea has been called the “weak underbelly” of the ice sheet because glaciers there are not buttressed by large floating ice shelves and so are most vulnerable to climate change. Disintegration of this sector, which alone contains enough ice to raise sea level by one meter, could trigger widespread retreat in the rest of the ice sheet. Worryingly, a recent study suggests that the rate of mass loss from these glaciers has more than doubled since 1996: they now account for nearly 70 percent of West Antarctic melt.

High-altitude glaciers in mountainous regions around the world are also retreating at an alarming rate. The World Glacier Monitoring Service in Zurich has tracked 30 glaciers from around the world since 1975. Between 1996 and 2005, these glaciers on average lost mass at twice the rate of the preceding decade, and four times the rate of the decade before that. If the current acceleration continues, large areas of many mountainous regions will lose all their ice in the coming decades. For example, Glacier National Park in Montana could lose the very features it is named for as early as 2030, and the Alps could be essentially ice-free by 2050. (See additional examples.)

Glaciers in the mountains of Asia nourish rivers that supply drinking and irrigation water to about 1.3 billion people. Without them, river flows could decline by up to 70 percent, meaning that some rivers would flow only during the rainy season. Currently, glaciers on the Tibet-Qinghai Plateau in China are retreating at an average of 7 percent a year, and as many as two thirds could disappear entirely by 2060. River systems that are at risk include the Yangtze, Yellow, Indus, Ganges, and Brahmaputra.

Glaciers in the Andes are also melting rapidly, a potential catastrophe for countries such as Bolivia, Ecuador, and Peru that depend heavily on meltwater for water supplies. Glaciers in Bolivia and Peru lost a third of their surface area between the 1970s and 2006, and continued loss could decrease river flows as early as 2030. In Peru’s coastal region, where 70 percent of the population resides, 80 percent of water resources come from snow and ice melt, which often constitutes the only source of water during the dry season. Peru’s major cities will face alarming water shortages in the near future if the glacial retreat continues.

Ice melt serves as a highly visible, early indicator of the effects of climate change, and recent years have seen the pace of melting accelerate, producing sea-level rise that is faster than had been predicted. Rising sea level today is already worrying for many small island nations and low-lying coastal areas that face severe flooding and possible submersion. The geologic record, however, shows us that ice sheets have collapsed catastrophically in the past, producing rates of sea-level rise up to 20 times faster than today. Such examples warn us to take seriously today’s rapid ice melt as a possible harbinger of more serious and devastating consequences if greenhouse gas emissions are not brought rapidly under control.